import java.util.HashSet;
import java.util.Set;

public class FindTheDifference {
    public static void main(String[] args) {
        String s  = "a";
        String t =  "aa";
        char ch = findTheDifference(s,t);
        System.out.println(ch);
    }

    public static char findTheDifference(String s, String t) {
        char ch = 0;
        for(Character c : s.toCharArray()){
            ch^= c;
        }

        for (Character c : t.toCharArray()){
           ch^=c;
        }
        return ch;
    }
}
